Quick Decision: Tg150 or Tg170?
| Criterion | Tg150 Acceptable | Tg170 Required |
|---|---|---|
| Layer count | 2-6 layers | 8+ layers |
| Lead-free reflow cycles | Single | Double or rework expected |
| PTH aspect ratio | < 8:1 | > 8:1 |
| Board thickness | < 1.6mm | > 1.6mm |
| Service temperature | < 130C | 130-150C |
| Via reliability class | IPC Class 2 | IPC Class 3 |
If your design hits ANY criterion in the Tg170 column, specify Tg170. The 15-25% cost premium is marginal insurance against field failures.
What Glass Transition Temperature Actually Means for Your Board
The Tg value on a laminate datasheet represents a phase transition, not a maximum operating temperature. Below Tg, the epoxy resin behaves as a rigid glass with predictable, low thermal expansion. Above Tg, the resin softens into a rubbery state where Z-axis expansion accelerates by 4-6x.
When your PCB enters a lead-free reflow oven with a 260C peak profile, the laminate temperature overshoots Tg by 90-110C for a Tg150 material. During those 60-90 seconds above Tg, the resin expands aggressively in Z while the copper barrel remains relatively stable. Each reflow cycle accumulates fatigue damage, particularly at the center of the thickest dielectric span.
A Tg170 material gives you 15% less expansion during this critical window, which compounds multiplicatively across multiple reflow exposures.
Thermal Property Comparison: The Numbers That Matter
| Property | Tg150 (Typical) | Tg170 (Typical) | Why It Matters |
|---|---|---|---|
| Tg (DSC) | 150C | 170C | Onset of accelerated Z-expansion |
| Td (TGA, 5% loss) | 320-330C | 335-345C | Decomposition onset in reflow |
| CTE Z (below Tg) | 42-48 ppm/C | 38-42 ppm/C | Normal service expansion |
| CTE Z (above Tg) | 250-280 ppm/C | 200-230 ppm/C | Reflow barrel stress driver |
| T260 | 20-30 min | 60+ min | Lead-free margin indicator |
| T288 | 10-15 min | 30+ min | Rework survival budget |
| Dk @ 1 GHz | 4.2-4.4 | 4.1-4.3 | Minimal signal impact |
| Cost multiplier | 1.0x | 1.15-1.25x | Laminate material only |
Two parameters matter far more than the Tg number itself: Z-axis CTE above Tg and T288 rating.
Z-axis CTE above Tg measures expansion once past glass transition. On a 2.0mm thick board, the 50 ppm/C difference between Tg150 and Tg170 produces approximately 5 microns less total Z-expansion per reflow cycle at 260C peak.
T288 quantifies how many minutes the laminate survives at 288C before delamination. Tg150 achieves 10-15 minutes; Tg170 delivers 30+ minutes. For boards requiring rework (each cycle adds 3-4 minutes of effective T288 exposure), the 3x improvement provides genuine engineering margin.
Via Reliability: Where Tg Selection Makes or Breaks Designs
The most common failure mode driven by Tg selection is plated through-hole barrel fatigue. When the laminate expands in Z during reflow, the copper barrel experiences tensile strain at the midspan of the thickest dielectric layer.
For a standard 1.6mm, 4-layer board with 0.3mm drills (aspect ratio 5.3:1), both Tg150 and Tg170 provide comfortable margin. The calculation changes dramatically for thicker constructions. A 2.4mm, 10-layer board with 0.25mm drills (aspect ratio 9.6:1) pushes Tg150 into marginal territory after a single reflow, and virtually guarantees barrel cracking if rework is attempted.
The IPC-6012 standard addresses this through minimum copper plating thickness requirements (25 microns average for Class 3), but plating thickness alone cannot compensate for excessive Z-expansion. When your aspect ratio exceeds 8:1 or thickness exceeds 1.6mm with more than 6 layers, Tg170 is the engineering-correct choice based on published fatigue data.
Cost Impact: Risk-Adjusted Analysis
The laminate cost premium for Tg170 over Tg150 runs 15-25% on the material, translating to roughly 8-12% on total fabrication cost. On production volumes above 1000 units in any non-consumer application, the premium functions as insurance with known, bounded cost.
One practical tip: specifying "FR-4, Tg >= 170C (DSC)" rather than a specific laminate brand gives your fabricator flexibility to use whatever Tg170-rated material is in stock, avoiding procurement delays and potentially reducing cost.
Application Decision Matrix
Specify Tg150 when ALL of these are true:
- 2-6 layers, thickness 1.6mm or less
- Single lead-free reflow with no planned rework
- PTH aspect ratio below 8:1
- Operating temperature below 130C
- IPC Class 2 reliability
Specify Tg170 when ANY of these apply:
- 8 or more layers
- Board thickness exceeds 1.6mm with any PTH
- Multiple reflow cycles or rework expected
- PTH aspect ratio exceeds 8:1
- Operating temperature 130-150C
- IPC Class 3 reliability
- Product lifecycle exceeds 10 years
How to Specify Tg in Fab Notes
Material: FR-4, Tg >= 170°C (DSC method per IPC-TM-650 2.4.25)
Compliance: IPC-4101/126 or /129 (halogen-free)
Avoid specifying just "high-Tg" (ambiguous), a brand without "or equivalent" (sole-source risk), or Tg by TMA without stating method (TMA reads 10-15C lower than DSC).
We've seen this cause confusion repeatedly in our facility—a designer writes Tg170 meaning DSC, the material cert shows 155 by TMA, and the incoming inspection flags it. State the measurement method or use IPC slash sheet numbers to eliminate ambiguity.
This article was originally published on the AtlasPCB engineering blog. We manufacture PCBs from 1 to 30 layers in both Tg150 and Tg170 FR-4, with controlled impedance and quick-turn options available.
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